Method of forming underground combustion front



VIRGN. J.' BERRY Jr.

INVENTOR. )PI e 4.7 AT TORN E Y V. J. BERRY, JR

Filed March 17, 1960 March 31, 1964 METHOD 0F FORMING UNDERGROUND COMBUSTIDN FRONT United States Patent O 3,126,956 ME'IHD 0F FURMING UNDERGROUND CMBUSTION FRUNT Virgil J. Berry, Jr., Tulsa, Okla., assigner to Pan American Petroleum Corporation, rIulsa, lrla., a corporation of Delaware Filed Mar. 17, 1960, Ser. No. 15,677 6 Claims. (Cl. 16E- 11) The present invention relates to an improved method for conducting underground combustion of carbonaceous materials. More particularly, it is concerned with a novel method for igniting an underground formation or formations Which are to be subjected to forward combustion, ie., a process in which the combustion front travels in the same direction as the injected air.

One of the difficulties encountered in igniting an underground formation prior to conducting combustion therein is concerned with the burners used. The application of such equipment necessarily involves either bringing to the well site sufficient gaseous or liquid fuel for a week or ten days operation, or making the required connections with a fuel source originating at the site. In remote areas where such combustion operations are usually carried out, transportation of the required fuel supply to the location frequently raises some difficult problems. Moreover, the use of such extraneous equipment to ignite the formation involves substantial additional expense. Also, serious explosions frequently result in the use of equipment of the above-mentioned type to effect ignition.

A further disadvantage, in addition to those caused by the use of bottom hole heaters for prolonged periods, is the fact that in forward combustion, considerable ditculty occurs in maintaining a uniform combustion front as it proceeds from the injection well toward the producing well. The maintenance of a balanced or uniform combustion front is particularly difficult where there are several oil-bearing zones involved which vary in respect to their permeability. In such cases, the largest portion of the injected air finds its way into the most permeable zone, causing a combustion front to be created in that zone, and to move out away from the well before the oil in the less permeable zones has had an opportnuity to ignite and become a part of a common combustion front traveling toward the producing well or wells. These conditions generally do not improve as the process continues, but on the contrary tend to become much worse, making it extremely difcult to produce the recoverable oil from the less permeable zones.

Accordingly, it is an object of my invention to provide an economical, safe and eliicient method for igniting an oil-bearing formation which is to be subsequently subjected to forward combustion. It is another object of my invention to provide a method for igniting a formation in which a minimum amount of extraneous fuel and heat are required to initiate combustion. It is a still further object to provide a method for igniting such formations wherein a highly combustible atmosphere is maintained in the well bore until all permeable oil-bearing zones, through which oxygen reaches said well bore, are lit. It is another object to provide a method for maintaining combustion in a well bore wherein the fuel for said combustion, after it has been initiated, is supplied solely from said formation.

In carrying out the process of my invention, a system of at least two wells is employed in a formation suiciently permeable to permit forward combustion. Adequate heat is furnished to a relatively small area of the oil-bearing formation penetrated by the injection well to heat said area to combustion supporting temperature, typically from about 1000 to about 1500 F. Meanwhile, air is injected into what is subsequently to be the producing well,

3,126,956 Patented Mar. 31, 1964 ice and ultimately reaches the injection well. With the passage of air through the formation, gaseous hydrocarbons and frequently liquid hydrocarbons are forced into the injection well along with air. As a result, we have a gaseous mixture of hydrocarbons and air contacting a small surface sufficiently hot to ignite the mixture in the well bore. Further introduction of air or oxygen-enriched air into the formation and into the well in this manner causes combustion to continue in the well bore as long as free oxygen reaches the latter. Wherever heat is liberated in the presence of oxygen, a combustion zone is formed and moves toward the entering air. This zone may range in temperature from about 500 to about 1500 F. because the air flux is high as the air flow converges into the well. Temperature in the well bore itself may be as high as 3000" F. due to burning of oil, coke and vapors. Generation of heat continues in the well until all points at which oxygen is entering have been heated suiliciently high to ignite the formation. Thereafter, the resultintg combustion zone moves toward the source of injected air. Heat liberated under such well bore conditions tends to provide uniform heating of the sand face iu contact with the burning taking place in the well bore. Even though the more permeable zones may tend to burn more readily, the rate at which they burn, of course, depends on the amount of air available, and since the injected air is diffused throughout the formation, there is no opportunity for high concentrations of it to be consumed by these zones. Accordingly, as the entire combustion front moves out in a reverse direction to the flow of air, it tends to become more uniform.

Ignition of all oil-bearing zones is indicated by a reduction in well temperature resulting from the fact that the entire combustion front is now moving toward the air source. After the front moves away from the well bore, which may require a period of from about 2 to about 24 hours, the temperature therein will be observed to drop from about 3000 F. to a value of from about 500 to about 1000 F. When this decrease in well temperature occurs, reverse air injection is halted, injection of air into the well in which combustion was initiated is begun, and thereafter the operation is carried on in the conventional manner employed in operating a forward combustion process.

To illustrate the process of my invention more specilically, reference is made to the accompanying drawing in which an injection well 2 and a producing well 4 penetrate an oil-bearing formation comprising zones 6, 6a and 6b. Each of these zones are separated by shale stringers 8. Casing 10 and 12 in wells 2 and 4, respectively, is run to the top of zone 6 and cemented to the surrounding formation at 14 and 16, leaving open hole from that level to the bottom of oil-bearing zone 6b. Both wells are equipped with a tubing string 18 and 21, and pipes 20, 22, 24 and 26, suitable for permitting the introduction or withdrawal of iluids.

In operating a system of this type, the Wells may be as much as feet, or more, apart. A source of heat, such as an open burner, chemicals, electrical heaters, a thermite bomb, or equivalent device, is placed opposite a small area which, e.g., need not be more than from about l to about 5 square feet, at the face of the oil-bearing zone 6a. The total amount of heat supplied in this fashion, if localized, may only amount to not more than about one million B.t.u. and may be considerably less since the object is to ignite the well bore mixture opposite the point of air efflux. In such procedure, injected air is supplied continuously from the formation in order to maintain combustion. While the small area in Well 2 is being heated, air is injected through pipe 24 into Well 4, then into zones 6, 6a and 6b, as indicated by the dashed arrows, and nally into the open hole of well 2. Although in some instances it may be preferable to ignite a small portion of the sand before injected air reaches well 2, I generally prefer to operate so that injected air is entering well 2 via zones 6, 6a and 6b prior to the time a maximum temperature is reached during the ignition process. To insure that such a condition does exist, I inject air via well l into Well 2 prior to igniting the formation. When oxygen is detected in the efllulent coming through pipe 20, and while injected air is still entering well 2, I purge the latter with air or a suitable inert gas in order to prevent the occurrence of an explosive mixture. After the purging step, the heat source is placed opposite an area, e.g., of from about l to about 5 square feet of the oil-bearing Zone, as previously described, and said area heated to a temperature of about 1200 to 1500 F. The oxygen necessary to sustian combustion is already being supplied, and gaseous hydrocarbons are entrained with the air entering the well bore to create a combustible mixture automatically and continuously. Contacting of this mixture with the heated portion of the formation serves to initiate and maintain combustion in the weli until the entire formation face has reached combustion temperature.

Referring again to the ignition of the face of the oilbearing formation, the hot surface at 6:1 spreads and increases in temperature owing to oxygen diffusing through zones 6, 6a and 6b. Air injection into the system in this manner is continued until the face of the formation at all three oil-bearing zones'has been lit, as indicated by combustion front 28. When this condition exists, no free oxygen can reach the well bore of well 2 and, hence, the tire that was burning therein is extinguished. This stage of the process is readily indicated by a drop of from about 3000" to not more than about l000 F. in well bore temperature, at which time pipes and 24 are closed and pipes 22 and 26 are opened. Air is then injected into pipe 22, typically at a daily rate of from about 20 to 25 thousand cubic feet per foot of oil-bearing formation, thereby causing combustion front 28 to move toward well 4, as indicated by the solid-lined arrows, forcingV gaseous and liquid hydrocarbons ahead of it.

It will be appreciated that the principles of my invention may be applied to advantagein an arrangement of wells much more complex than that illustrated. The fact is to be stressed that this invention concerns a method for igniting any formation in which the forward combustion process can be employed. Also, it should be recognized that numerous vari-ations in procedure may be utilized without departing from the scope of my invention. For example, instead of injecting air into the well bore via an offset well, air may be introduced under high pressure, typically 500 to 1000 p.s.i., into the well in which the formation is to be ignited. At the same time, the remaining wells in the affected area are shut in. After a suiiicient'back pressure has been built up in the formation to cause theV well on backow to produce air for several hours at the anticipated injection rate, localized heating is applied, as previously described, over a small area or" the formation. Typically, such back pressure should be in a range of from about 500 to about 1000 p.s.i. When the heated portion of the formation reaches the temperature of about 1200 to about 1400 F., heating is discontinued and Vpressure on well 2 of the drawing is slowly decreased, causing a back flow of air and combustible gas into the well bore. When this mixture of air and gas contacts the hot formation, ignition occurs. Burning of the air-gas mixture continues as long as free oxygen reaches the well bore. When this burning ceases, the formation has been lit and the combustion front proceeds away from the well bore. At this time, the ,remaining wells are opened and air is injected into the well in which combustion was initiated. From this point on, forward combustion is continued in the usual fashion. It will be appreciated that this particular modiiication of my invention should, in general, be limited to formations that 'are suliiciently permeable (in terms of gas space) to permit injection of a volume of air adequate, when permitted toA backflow, to cause complete ignition of the oil or tar bearing sands in the bore of well 2. In this connection, it is to be understood that the terms permeable or permeable zones, as used in certain of the appended claims, are to be construed in light of, and are to be limited to, the comment immediately above.

Actually, to employ the principles of my invention, the formation face need not be heated to initiate cornbustion. If desired, an open burner, electrical heater or other suitable device, may be used to light the combustible air-gas mixture as it comes from the formation, after which the ignition device may be withdrawn from the well. Also, while in many cases the combustible mixture forced into the well bore during ignition of the formation consists essentially of air and natural or other combustible gas, it may contain hot crude oil or the equivalent, or it may be essentially air and hot crude oil.

The term carbonaceous as used herein is intended to refer to materials comprising either free or combined carbon.

I claim:

l. In a process for forming a combustion front in an underground carbonaceous deposit penetrated by an injection well and a producing well, the improvement which comprises heating a portion of said deposit at the face of said injection well to a combustion supporting temperature whereby carbonaceous material on the face of said well is vaporized, injecting into said deposit via said producing well a gas selected from the group consisting of air and oxygen-enriched air, forcing said gas through said deposit into said injection well to form a combustible mixture of said gas and said vaporized carbonaceous material in said injection well, allowing said mixture to contact said heated portion whereby said mixture is ignited, continuing the injection of said gas into said producing well until the temperature of said injection well has passed through a maximum indicating a combustion front has been formed on the face of said injection well and is moving away therefrom, discontinuing injection of said gas into said producing well and thereafter introducing an additional quantity of said gas into said injection Well whereby said gas and said front travel through said deposit in the same direction.

2. In a process for forming a combustion front in an underground carbonaceous deposit penetrated by an injection well and a producing well, the improvement which comprises heating a portion of said deposit at the face of said injection well to a combustion supporting temperature whereby carbonaceous material on the face of said well is vaporized, injecting into said deposit via said producing well a gas selected from the group consisting of air and oxygen-enriched air, forcing said gas through said deposit into said injection well to form a combustible mixture of said gas and said vaporized carbonaceous material in said injection well, allowing said mixture to contact said heated portion whereby said mixture is ignited, continuing the injection of said gas into said producing well until the temperature of said injection well has decreased to a tlevel not more than about 1,000 degrees F., discontinuing injection of said gas into said producing well and thereafter introducing an additional quantity of said gas into said injection well whereby Vsaid gas and said front travel through said deposit in the same direction, and recovering fluids through said producing well resulting from the cocurrent travel of said gas and combustion front through said deposit.

3. The process of claim l in which said gas is air.

4. In a process for forming a combustion front in an underground carbonaceous deposit penetrated by an injection well and a producing well, the improvement which comprises heating not more than from about 1 to about 5 square feet of the face of said deposit penetrated by said injection well to a temperature of from about 1,000 to about 1,500 degrees F. whereby carbonaceous material on the face of said injection well is vaporized, injecting into said deposit via said producing well a gas selected from the group consisting of air and oxygenenriched air, forcing said gas through said deposit and into said injection well to form a combustible mixture of said gas and said vaporized carbonaceous material in said injection well, allowing said mixture to contact the heated portion of said face whereby said mixture is ignited, continuing the injection of said gas into said producing well until the temperature in said injection well has decreased to a level of not more than about 500 degrees to about 1,000 degrees F., indicating a combustion front has been formed in said injection well and is moving away therefrom, discontinuing injection of said gas into said producing well, thereafter introducing an additional quantity of said gas into said injection well whereby said gas and said front travel through said deposit in the same direction, and recovering uids through said producing well resulting from the cocurrent travel of said gas and combustion front through said deposit.

5. In a process for forming a combustion front in an underground hydrocarbon reservoir containing gaseous hydrocarbons, and penetrated by an injection well and a producing well, the improvement which comprises heating a portion of said reservoir at the face of said injection well to a combustion supporting temperature whereby a portion of the hydrocarbons on the face of said injection well is vaporized, injecting into said deposit via said producing well a gas selected from the group consisting of air and oxygen-enriched air, forcing said gas through said reservoir where it is mixed with said gaseous hydrocarbons to form a combustible mixture in said reservoir, forcing said mixture into said injection well by the continued introduction of said gas into said producing well, allowing said mixture to contact said heated portion whereby said mixture is ignited, continuing the injection of said gas into said producing well until the temperature of said injection well has passed through a maximum indicating a combustion front has been formed on the face of said injection well and is moving away therefrom, discontinuing injection of said gas into said producing well and thereafter introducing an additional quantity of said gas into said injection well whereby said gas and said front travel through said deposit in the same direction.

6. The process of claim 5 in which said gas is air.

References Cited in the file of this patent UNITED STATES PATENTS 2,858,891 Moll et al. Nov. 4, 1958 2,880,803 Parker Apr. 7, 1959 2,917,112 Trantham et al. Dec. 15, 1959 2,994,374 Crawford et al. Aug. 1, 1961 2,994,376 Crawford et al. Aug. 1, 1961 3,007,521 Trantham et al. Nov. 7, 1961 3,010,512 Hurley et al, Nov. 28, 1961 

1. IN A PROCESS FOR FORMING A COMBUSITON FRONT IN AN UNDERGROUND CARBONACEOUS DEPOSIT PENETRATED BY AN INJECTION WELL AND A PRODUCING WELL, THE IMPROVEMENT WHICH COMPRISES HEATING A PORTION OF SAID DEPOSIT AT THE FACE OF SAID INJECTION WELL TO A COMBUSTION SUPPORTING TEMPERATURE WHEREBY CARBONACEOUS MATERIAL ON THE FACE OF SAID WELL IS VAPORIZED, INJECTING INTO SAID DEPOSIT VIA SAID PRODUCING WELL A GAS SELECTED FROM THE GROUP CONSISTING OF AIR AND OXYGEN-ENRICHED AIR, FORCING SAID GAS THROUGH SAID DEPOSIT INTO SAID INJECTION WELL TO FORM A COMBUSTIBLE MIXTURE OF SAID GAS AND SAID VAPORIZED CARBONACEOUS MATERIAL IN SAID INJECTION WELL, ALLOWING SAID 